Contrary to popular belief I'd say those sizes usually lean small but not flat, so unless you think you're literally board flat or rather large, I think you shouldn't be too far off as a starting point. Sometimes people have personal comfort preferences for a size up or down, but you should at least have the right ballpark.
I know a lot of media says DDs and DDDs are huge, but it's a huge misconception perpetuated by an scummy industry and porn/sexualization. The thread I posted talks about it a bit, where women that should be in stuff like GG get put in DDs so it skews people's perception, and actual DDs are put in like B.
This is a website where you can look at non-sexual images of women in different sizes, and also read reviews on specific bras to learn its properties https://www.bratabase.com/browse/by-photo/32:5/
Re: cleavage, if you wear a fitting bra you USUALLY shouldn't have a clean sharp line of cleavage. A lot of advertising pushes the idea that it's sexy to have it but it's just sugarcoating the fact that they're putting people in the wrong size, as aforementioned. That's just one way they try to manipulate women to buy their stuff even though it sucks.
In this blog post you can see that even though this woman has large breasts, she doesn't have the line when wearing a regular fitting bra and only gets varying degrees of it with specific bras that were intentionally designed for cleavage or by wearing stuff that's technically too small. Nothing should be spilling out of the cups in any direction, they should be big enough to mostly encapsulate the breast, not just laying on top of them. https://obsessedwithbreasts.wordpress.com/2012/12/05/cleavage-concerns/
Also your shape sounds a lot like mine so I get what you mean! The terminology that that calculator uses is "wide set" and "full on bottom." Like I said, for me I look pretty small when I don't wear a bra but it makes a good difference when I do wear one. The breast tissue is still there, it's just not up and forwards so you don't notice it until you bring it together that way. My size isn't the same as yours but it's a sister size which means the same volume, just a different band.
A tip that's good for everyone is that when you put on a bra, you should do something called "scoop and swoop" to make sure you're wearing it properly. If you look it up you can probably get a better visual than by me trying to describe in text. Basically a lot of women don't put all their tissue in the cup and they're mashing some tissue towards the sides, which can lead to discomfort or an unflattering shape. Sometimes you don't notice a bra is too small until you do this. I think that's why a lot of bra stores don't teach you this… It masks when a bra doesn't fit. Here's a blog post that explains in more detail with example images: http://brasandbodyimage.com/2012/09/05/putting-your-bra-on-properly/